Proceses and apparatus for reducing electrode wear in a plasma arc torch

ABSTRACT

A process and apparatus for reducing wear of an electrode in a plasma arc torch involves altering the gas flow in a plasma chamber surrounding the electrode immediately before and continuing after cutting of the current that sustains the arc. The altering includes closing off the gas flow upstream of the chamber, switching from a swirling flow to a radial/axial flow through the plasma chamber, reducing the arc current level in conjunction with either of the above, and venting the plasma chamber to rapidly change the gas flow and pressure in the chamber. The interval is sufficient to reduce electrode wear, but short enough that the arc remains stabilized until cut-off. In the flow stop mode, a solenoid valve is placed on an inlet tube for the plasma gas. For venting, a vent tube and another solenoid valve are added to the torch. In the flow pattern switching mode, two independent gas supply lines with control valves feed separate sets of gas inlets to the plasma chamber, one set producing a swirling flow and the other set producing a radial then axial flow.

BACKGROUND OF THE INVENTION

This invention relates in general to plasma arc cutting and weldingprocesses and apparatus. More specifically, it relates to a process andapparatus for reducing electrode wear.

Plasma arc torches have a wide variety of applications such as thecutting of thick plates of steel and the cutting of comparatively thinsheets of galvanized metal commonly used in heating, ventillating andair conditioning (HVAC) systems. The basic components of a plasma arctorch include a torch body, an electrode (cathode) mounted within thebody, a nozzle (anode) with a central exit orifice, a flow of anionizable gas, electrical connections, passages for cooling, and arccontrol fluids, and a power supply that produces a pilot arc in the gas,typically between the electrode and the nozzle, and then a plasma arc, aconductive flow of the ionized gas from the electrode to a workpiece.The gas can be non-reactive, e.g. nitrogen, or reactive, e.g. oxygen orair.

Various plasma arc torches of this general type are described in U.S.Pat. Nos. 3,641,304 to Couch and Dean, 3,833,787 to Couch, 4,203,022 toCouch and Bailey, 4,421,970 to Couch, 4,791,268 to Sanders and Couch,4,816,637 to Sanders and Couch, and 4,861,962 to Sanders and Couch, allcommonly assigned with the present application. Plasma arc torches andrelated products are sold in a variety of models by Hypertherm, Inc. ofHanover, New Hampshire. The MAX 100 torch of Hypertherm is typical ofthe medium power torches (100 ampere output) using air as the workinggas and useful for both plate fabrication and HVAC applications.

In all plasma arc torches, a common and heretofore unsolved problem hasbeen a substantial wear of the electrode, particularly when theelectrode is used with reactive gases such as oxygen or air. Forexample, the standard electrode for the MAX 100 brand torch ofHypertherm, Inc. shows wear as a generally concave pit on the lower endof the electrode, or more precisely, on an emitting element of hafniummounted on the electrode. On average a wear depth of about 0.025 inch isobserved in such a Hypertherm brand electrode after 120 cut cycles. Thewear results of commercially available units of others, as measured byHypertherm, Inc., are typically worse.

For the MAX 100 brand torch, when the wear produces a pit depth of 0.060inch or more, Hypertherm recommends that the electrode be replaced. Inordinary use, the electrode of a plasma arc cutting torch operating withreactive gases typically requires replacement after 0.5 to 2 hours ofuse depending strongly on the number of on off cycles. Wearconsiderations are significant not only because they necessitate therepeated replacement of a component, but also because they limit themaximum power that can be applied to a given torch.

In plasma arc cutting, it is also important to note that the quality ofthe cut is highly dependent on the flow pattern of the gas in a plasmachamber, defined at least in part as the region between the electrodeand the nozzle. In particular, a swirling flow produced by injecting thegas tangentially into the plasma chamber has been found to be essentialto produce a high quality cut. A swirling gas flow pattern is alsoimportant in stabilizing the plasma arc so that it exits the torch toattach to and cut the workpiece but does not contact the torch nozzleitself. The nozzle is the principal component that is damaged by the arcwhen the arc is not well controlled.

Another design consideration is the very high temperatures of theplasma, e.g. greater than 10,000° C. These temperatures introducecorresponding changes in the gas properties such as its density andviscosity. These considerations are significant on start up and cut-off.On start up the arc rapidly heats the gas which significantly decreasesthe gas density exiting the nozzle orifice. This presents the situationwhere the gas flow is choked in the nozzle orifice region. On cut-off,the situation reverses and there is a tendency for the gas in the plasmachamber to blow out of the chamber very suddenly as the mass flowincreases when the temperature drops.

It is common industry practice to use hafnium or zirconium as thecathodic emitter insert in the electrode. Hafnium as of today, is thebest choice for the cathodic emitting element when cutting with areactive gas plasma. It exhibits the least wear of all other materialstried for this application, but is more costly than other materials.These electrodes nevertheless require frequent replacement. Lower wearhas been associated with lower current levels, but at some point thereduction in performance associated with a reduced operating currentbecomes too great. Cooling the electrode has also been used to increaseelectrode life, whether by way of a gas flow or water flow placed ingood thermal communication with the electrode. However, water cooling isexpensive, cumbersome and is not desirable for low current units e.g.those rated below 100 amperes. Air cooling is less efficient and limitsthe maximum operating current of the torch, even one carrying acomparatively low current. Therefore, to date, the only practicalsolution to the electrode wear problem has been to replace the entireelectrode again and again, despite the clear economic disadvantages ofthis approach.

It is therefore a principal object of the present invention to reducethe wear on the electrode of a plasma arc torch significantly andthereby extend its life.

Another principal object of this invention is to reduce electrode wearand thereby allow operation at higher current levels than are presentlyfeasible, even when operating with reactive gases.

Another object of the invention is to achieve a better cut quality thanhas heretofore been possible by allowing a greater level of swirl.

Another object of the invention is to reduce the electrode wear in amanner so as to allow the substitution of the hafnium emitting elementwith other less expensive materials such as zirconium.

A further object of the invention is to provide the foregoing advantageswhile using standard electrode and nozzle constructions and without anysignificant increase in the incidence of damage to torch parts such asnozzle gouging.

Yet another object of the invention is to provide the foregoingadvantages for existing plasma arc torch systems using onlycomparatively simple and inexpensive modifications.

A still further object is to provide the foregoing advantages at afavorable cost of manufacture and operation.

SUMMARY OF THE INVENTION

A plasma arc cutting torch, particularly one using a reactive gas andemployed in cutting metallic materials, has a torch body that mounts anelectrode and a nozzle in a spaced relationship to define a plasmachamber therebetween. An ionizable gas is fed by tubes, passages andchambers to and through the torch body to a swirl ring mounted in thetorch body. The swirl ring feeds the gas to the plasma chamber where itis ionized and exits the torch via a central exit orifice formed in thenozzle. The torch also includes standard electrical connections to apower supply and an electrical controller to initiate a pilot arc in thegas in the chamber and then transfer the arc to a workpiece for cuttingor other operations.

A principal feature of the present invention is the process step ofaltering the mass flow rate of the gas and/or its flow patternimmediately before and immediately after the step of cutting off of thecurrent to the torch. The mass flow rate is reduced by reducing the gasflow, by closing off or reducing the gas flow to the plasma chamber Thismass flow rate reduction is timed to occur within a few hundredmilliseconds before the current cut off, and preferably continue aftercut off. The process step of cutting the arc current can be accomplishedby a sudden step function of time or a gradual ramp function of time.The reduction in the gas flow may be coupled with a venting of theplasma chamber to atmosphere to facilitate a more rapid change in thegas flow pattern in the plasma chamber.

Alternatively, this invention includes altering the gas flow patternfrom a standard swirling flow with a tangential component to one with agenerally radial flow at its input to the plasma chamber, and then agenerally axial flow through the chamber. This switch from a swirlingflow to a radial/axial flow may be in conjunction with an overallreduction in the flow rate, and in conjunction with a decline in thecurrent level and/or a venting of the plasma chamber to atmosphere. Ithas been found that by substition to a more axial flow a higher overallmass flow rate can be tolerated at the time just preceeding current offthan with a swirling flow.

When there is no venting, which is the preferred case, the gas pressurehas been found to decay generally exponentially and the aforementionedreductions in mass flow preferably occur within about 500 millisecondsof start of the current cut off step. The preferred current off step isa controlled linear decreasing ramp which follows the decrease in massflow. This ramp would continue to very low values of current, until thearc goes out. This preferred plasma off process provides the desiredreduction in electrode wear while still providing sufficient gas flow tostabilize the plasma arc and avoid damage to the nozzle. With venting,the gas flow through the plasma chamber and thus the gas pressure in theplasma chamber decay much more rapidly. However the precise optimaltiming will vary with the application since it is a function of multiplevariables, including gas type, current level, size of nozzle orifice,inlet flow area of swirl ring, gas pressure, gas flow rate, gas flowpattern (e.g. swirling or axial), and the physical distance between theplasma chamber and valving controlling flow conditions just prior tocurrent cut-off.

Viewed as an apparatus, the invention includes, as a flow alteringmeans, a valve in a line supplying gas to the plasma chamber that can beclosed just prior to current cut off by a standard controller of a powersupply. This flow rate reduction may be in coordination with a ramp downof the arc current. The reason for the current ramp down is to reducethe possibility of damage to the nozzle orifice during the time of lowmass flow just prior to current off. In an alternate form, the vent linefrom the plasma chamber to atmosphere is controlled (opened and closed)by a valve. The valve is opened by the controller in the critical periodjust prior to current cut off. In another form, the apparatus includestwo swirl rings or two separate positions of the same ring, one whichproduces a conventional swirl flow and a second which producesprincipally an axial flow without swirl. These rings may be mounted inan axially stacked array. These swirl rings are connected to a gassupply by independent feed lines at least within the torch body, and avalve, or valves, switches the gas flow just prior to cut off, inresponse to the controller, from the swirl inducing ring or ring portionto the radial/axial flow ring or ring portion. This switching may be inconjunction with venting and/or a flow rate reduction, and/or a currentlevel reduction.

These and other features and objects of this invention will be morefully understood from the detailed description which should be read inlight of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified schematic view of a typical plasma arc torchconstructed to operate according to the present invention;

FIG. 1B is a view in horizontal section along the line 1B--1B in FIG.1A;

FIG. 1C is a simplified schematic view of a plasma arc cutting systemaccording to the present invention using the torch shown in FIGS. 1A and1B;

FIG. 1D is a six graph timing diagram of the gas flow alteration of thepresent invention in relation to the cut-off arc current;

FIGS. 2A, 2B and 2C correspond to FIGS. 1A, 1B and 1C and show analternative embodiment of the invention utilizing a valved vent incombination with a valved gas feed; and

FIGS. 3A, 3B, 3C, and 3D correspond to FIGS. 2A-2C, FIGS. 3B and 3Ccorresponding to FIG. 2B, and show an alternative embodiment of theinvention utilizing axial and radial inlet hole sets in a swirl ring toestablish either swirled or axial gas flow patterns in the plasmachamber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B show in simplified schematic form a typical plasma arccutting torch 12 such as any of a variety of models of torches sold byHypertherm, Inc. The present description is therefore intended toillustrate the basic principles and elements common to plasma arccutting torches of this type, but not to describe construction detailsof any particular torch model. The torch has a body indicated generallyat 14 which typically is cylindrical with an exit orifice 16 at one end,the lower end 12a as shown, through which a plasma arc 18, an ionizedgas jet, passes and attaches to a metal workpiece 20 which is being cut.The gas can be non-reactive, such as nitrogen or a noble gas, but it canalso be reactive, such as oxygen or air. A significant advantage of thepresent invention is that the torch can operate with a reactive gas andnevertheless have dramatic improvement in electrode wear as compared tothe best results obtainable with any plasma arc cutting torch,regardless of the current level, or the effectiveness of the torchcooling arrangements.

The torch body 14 supports a cathode 22, commonly referred to as anelectrode, which is typically made of copper and has a generallycylindrical configuration. An emitting element 24 is press fitted intothe lower end face of the electrode 22. The electrode and the emittingelement are centered within the torch body and aligned with the exitorifice 16. When using a reactive gas, the insert is typically formed ofhafnium or zirconium. The body 14 also mounts a nozzle 26 with a centralnozzle orifice that defines the torch exit orifice 16. The nozzle isspaced from the electrode. A swirl ring 28 having a set of radiallyoffset gas distribution holes 30 is replaceably mounted in the torchbody. It is positioned to feed a flow of the plasma gas from the holes30 to a plasma arc chamber 32 defined, as shown, by the electrode, thenozzle and the swirl ring. The offset of the holes, best seen in FIG.1B, introduce a tangential velocity component to the gas flow throughthe chamber 32 causing it to swirl. An advantage of this invention isthat the level of swirl introduced by these holes can be greater thanhas heretofore practicable resulting in a better cut quality than hasheretofore been attainable. The swirl ring is shown in a tight fitting,gas sealed relationship to the electrode. It will be understood,however, that the swirl ring is typically mounted in a support memberand does not itself seal to the electrode. More generally, theparticular construction details of the torch body and arrangements formounting these elements directing gas and cooling fluid flows andproviding electrical connections can and do take a wide variety offorms.

A negative power lead 34 surrounds the upper end of the electrode 22 tomake a good electrical connection. A retaining cap 36 threads onto acurrent ring 38 that forms the upper end of the torch body 14. Aninsulator sleeve 40 separates and electrically isolates the current ring38 from the negative power lead 34. The retaining cap has a lower lip36a which engages a flange 26a on the nozzle in an abuttingrelationship. The retaining cap when tightened onto the ring 38 capturesand replaceably secures the nozzle against the swirl ring. In analternative construction not shown, it can also capture and secure theswirl ring between the nozzle and another internal support structurewithin the torch body. In the configuration shown, the retaining cap 36also, in part, defines a gas plenum chamber 42 that acts as a localsupply to the swirl ring 28 and the plasma chamber 32. A flow 44 ofplasma gas passes through an inlet tube 46 which penetrates the currentring 38 to feed the gas to the plenum chamber 42. A solenoid valve 48such as Model No. AFP33183 manufactured by Automatic Switch Company issecured in the inlet tube, preferably at a point closely spaced from thetorch body. Control signals to the valve over lines 48a,48a open andclose the valve to regulate the flow rate of the gas to the torch. In apilot arc mode of operation, where a pilot arc is drawn between theelectrode and the nozzle through the gas in the plasma chamber, thenozzle acts as an anode and the cap 36 and current ring 38 form a pilotarc current return circuit.

In operation, plasma gas 44 flows through the solenoid valve 48 and theinlet tube 48 into the plenum chamber 42. From there, it flows throughthe gas distribution holes 30 in the swirl ring 28 into the plasma arcchamber 32 and finally out of the torch through the nozzle orifice 16.When the torch is operating in the nontransferred pilot arc mode, apower supply 50 provides current to the torch at an appropriate voltageto initiate the pilot arc and then maintain it. The power supply can beany conventional regulated D.C. supply and includes a p.c. board or thelike which controls the operation of the power supply and othercomponents of the torch system such as flow control valves.

With reference to the complete plasma arc cutting torch system 52depicted in FIG. 1c, the complete current path in the nontransferredpilot arc mode is from the negative power supply terminal 50a, throughlead 54, the negative power lead 34, electrode 22, a pilot arc plasma(FIG. 1A), the nozzle 26, the retaining cap 36, the current ring 38, apilot arc return lead 58, a closed pilot arc switch 60, and a powersupply positive terminal 50b. When the torch 12 is lowered toward theworkpiece 20, the arc transfers to the workpiece as the ionized plasmajet 18. This allows some current to flow from the workpiece through aground lead 62 back to the power supply positive terminal 50b. When thistransferred current path is established, the pilot arc switch opens andthe torch is cutting the workpiece. In this transferred or cutting mode,the current path is from the power supply negative terminal 50a, thelead 54, negative power lead 34, electrode 22, the plasma arc or jet 18,the workpiece 20, the ground lead 62 and the power supply positiveterminal 50b.

An operator sets a desired gas flow or pressure prior to initiating thepilot arc at a control console 64 which is removed from the torchitself. The console includes gas flow regulators and gas valvingpressure gauges. The flow and pressure values set by the operator at theconsole correspond in a known wag to the actual gas flow and pressure inthe plasma chamber 32 prior to the pilot arc. Starting the pilot archeats the gas causing an increase in the gas temperature within thechamber and a decrease in the flow, in a manner well known in the art. Atypical gas pressure in the chamber 32 during the pilot arc is 20 to 40psi. The pilot arc is started by a high frequency spark or other means,such as a contact starting technique, all of which are well known in theart. During start up the plasma gas flows through the tube 46, solenoidvalve 48, plenum chamber 42, swirl holes 30, the plasma chamber 32 andout the exit orifice 16. As noted above, the swirling flow establishedby the holes 30 is very important in obtaining a good quality cut and instabilizing the arc within the nozzle exit orifice 16--to prevent thearc from impinging on the nozzle and gouging it.

Also as noted above, the torch begins cutting when it is brought closeto the workpiece so that the arc transfers to the workpiece and thecontroller opens switch 60. After transfer, in normal operation thecontroller increases the current level to a predetermined value forcutting. This increase in current also results in an increase in theheating of the plasma gas, a further increase in the gas pressure in theplasma chamber, and a further decrease in the gas flow out of the nozzleexit orifice. The maximum recommended current levels vary greatly amongdifferent torches and applications, with currents in the range of about20 to 200 amperes D.C. being characterized as low currents and those 200and above being high currents. A typical current level for a watercooled cutting torch used to cut plate steel is 400 amperes.

A principal discovery of the present invention is that most of the lossof material (wear) of the electrode during its operation occurs notduring start up or during the actual cutting, but rather when thecurrent to the arc is shut off. While the mechanisms for this wear arenot fully understood, there is evidence that the electrode becomesmolten, at least in part, during operation and that on cut off of theelectrical power wear is related to a complicated interaction betweenthe molten surface(s) of the electrode and the flow and pressure of theplasma gas through the plasma chamber.

A principal feature of the present invention is a control of the plasmagas flow to the plasma chamber in conjunction with a controlledelectrical shut off to reduce electrode wear substantially. In itssimplest form, the invention involves a total shut off of the plasma gasflow to the chamber 30 just before (1) a total, step function shut offof the arc current or (2) at the same time the arc current starts agradual shut off, but also just before a total shut off of the arccurrent. This effect is achieved by closing the solenoid valve 48 justprior to the total shutting off process of the arc current. Thepreferred process for shutting off the current is a controlled lineardecreasing ramp which follows the decreasing mass flow. The timing ofthese shut offs is, however, critical. If the gas flow is allowed todecrease rapidly, there is a significant diminution of the swirlingplasma flow that stabilizes the arc. Therefore the arc can and willattack and damage, or even destroy, the nozzle in a very short time. Onthe other hand, if the shut-offs are too close in time, the gas flow andpressure in the chamber 30, which decay in a generally exponentialmanner with the valve 48 shut, exhibit little change and wear occurs toabout the same extent as if the valve 48 was left open. The gas flowshut off continues through the arc current shut off, and thereafter.

The timing of the gas and current shut off processes in accordance withthe present invention are illustrated by the timing diagrams of FIG. 1D.The three lefthand graphs show the control signals (whether a voltagesignal, current signal, or otherwise) as a function of time. The firstlefthand graph shows a control signal (applied over lines 48a,48a) tothe solenoid valve 48 changing its state at a time t₁ (the signal goesfrom a "1" state to a "0" state which are indicative of either a digitalswitching or an analog change sufficient to produce the desired changein state of the valve 48). This change in the control signal closes thevalve at t₁. The second lefthand graph shows a control signal for thearc current, a signal generated by the controller of the power supply50. In accordance with this invention, the arc current control signalchanges its state, again shown as a change from a "1" state to a "0"state, at a time t₂ which is after time t₁. The gas flow is thereforecut off before the arc current by a time interval Δt equal to thedifference between t₂ and t₁. The third lefthand graph shows analternative control sequence for the arc current cut off process. Inaccordance with this invention the arc current control signal changesits state, again shown as a change from a "1" state to a "0" state, at atime t₁ which is the same time as the valve control signal. This changecommences the operation of the controller to produce a ramp down of thearc current as shown in the lowermost righthand graph of FIG. 1D.

The righthand graphs in FIG. 1D show: 1) the gas flow rate through theplasma chamber 32; 2) the arc current shut off process as a sudden stepoff at t₂ ; and 3) an alternative arc current shut off process as agradual linear ramp down from full current at t₁ to a minium sustainablecurrent at t₂ when the arc current snuffs out to zero. These graphs arealso presented as a function of time and for the same periods of time asthe corresponding lefthand graphs. After closing the valve 48 at t₁, thegas flow falls steadily. A generally linear fall off in flow is shown,but the relationship is actually more complex and the curve is in factgenerally exponential. The important factor is that the gas flow valuefalls substantially over the interval Δt so that a) it is at acomparatively low value at t₂ when the arc current is shut off, asillustrated by the second righthand graph of FIG. 1D, or b) it is acomparatively low valve at t₂ following the current ramping downillustrated by the third righthand graph of FIG. 1D. The presentlypreferred embodiment of this invention uses non vented ramp down of thegas flow as shown in the upper righthand graph in combination with anarc current ramp down as shown in the lower righthand graph that followsthe change of the gas flow over the time interval Δt. This preferredmode of operation seems to produce the least wear despite the fact thata lower arc current changes the gas density in a manner that produces anincreased gas flow rate through the plasma chamber--other factors beingconstant.

While the precise value of Δt varies with each torch and the particularoperating parameters, for most low current plasma arc cuttingapplications a Δt of 500 milliseconds or less has been found to be theright timing to reduce electrode wear. For the MAX 200 brand torch, a Δtof roughly 250 to 300 milliseconds without venting and with a followingcurrent ramp down has been found to be optimal. In the operation of aMAX 100 brand torch after 120 cut cycles using this invention there is apit depth (wear) of about 0.005 inch, whereas normal operation withoutusing this invention produces a wear depth of about 0.025 inch in thesame electrode insert 24. This wear reduction translates into anelectrode life which is five times the best value that has ever beforebeen attainable. This invention may also allow, on average, torches tobe operated at powers in excess of their conventional ratings.

Note that at t₂ there is still a residual gas flow even though the gasfeed is cut off at t₁. This ensures that until and at current cut offthere is a sufficient flow in the chamber to stabilize the arc andprevent nozzle damage. Also, there is a brief surge in the flow aftercut off of the current. This is believed to reflect a sudden cooling ofthe gas in the absence of the arc and a rapid out flow of gas from thetorch driven by the gas pressure in the plasma chamber and the suddenchange in the properties of the gas after the arc is extinguished. Thisphenomena suggests that while the gas flow to the torch can be reducedover the interval Δt by reducing the flow 44 to the plasma torch, that acomplete closure of the valve 48 is preferred since this closureupstream of the plasma chamber dampens the strength of the flow surgeand limits the total volume of the flow when the current is cut off. Asalready stated, it is also within the scope of the present invention toreduce the current at t₁, e.g. by ramping it down over the interval Δtrather than having an abrupt shut off at t₂. It is also under the scopeof this invention to allow a reduction in the overall current prior tot₁, or after t₁, i.e., prior to or after closing the solenoid valve.

FIGS. 2A-2C show the plasma arc cutting torch 12' incorporating analternative embodiment of the invention, like parts in the FIG. 2A-2Cembodiment having the same reference numbers as in FIGS. 1A-1C, but witha prime. The structure and mode of operation of the torch 12' and torchsystem 52' is the same as described above with respect to the FIGS.1A-1C embodiment, except for the addition of a vent tube 66 and anassociated solenoid valve 68 connected in the vent tube to open andclose it. The tube 66 penetrates the current ring 38' and is in fluidcommunication with the plenum chamber 42'. A control signal from thecontroller carried over lines 68a,68a operates the valve 68. In thisembodiment, when the solenoid valve 48 is closed at time t₁. The ventvalve 68 is opened. Because the vent tube 68 is open at its end 66a atatmosphere, or to some other lower pressure region such as a vacuumchamber, opening the valve 68 causes the gas flow and pressure in theplenum and the plasma chamber to decay more rapidly than the decay ofthe FIGS. 1A-1C embodiment. This allows the current to be shut off morequickly after the gas flow is cut-off at time t1. It has beendiscovered, however, that timing is very important in thisconfiguration. Since by venting an alternative flow path is established,the flow through the nozzle can go to low values and cause the plasma tobecome unstable very quickly. In general, when venting is used the flowalteration preceeds the arc current shut off by a significantly shortperiod of time and without venting. With this venting, the interval Δtcan be reduced from about 250 milliseconds to less than 5 millisecondswhen operating a MAX 200 brand torch with air. This may reduce thelikelihood of nozzle damage caused by a destabilized arc. It is alsocontemplated that valves 48 and 68 can be combined in a single ventingtype valve.

FIGS. 3A-3D show another embodiment of a torch 10" and torch system 52"utilizing according to the present invention, like parts beingidentified with the same reference numbers, but double primed. Thisembodiment utilizes the discovery that electrode wear can be reducedsubstantially if the gas flow through the plasma chamber is changed notonly in flow rate, but also in flow pattern, just prior to current shutoff. More specifically, electrode wear is reduced to almost negligiblelevels on current shut off if the degree of swirling of the gas isreduced just before cut-off. At moderate gas pressures, this resultholds for even high gas flow rates (e.g. 120 scfh). In operation withthe Hypertherm® MAX 100 brand torch, negligible wear was observed whenthe gas flow into the plasma chamber was radial (no swirl) and the gaspressure in the chamber was below 30 psi. While a perfectly radial flowand moderate to low gas pressures produce the best results, theinvention also provides reduced electrode wear with less than aperfectly radial flow and at increased gas pressures. In this embodimenta major concern once again is destabilization of the plasma arc in theabsence of a swirling flow. The solution of the present invention is touse a swirling flow, and then suddenly switch to a radial flow, withsubstantially- no interruption of the overall flow rate, immediatelybefore current cut off. The torch 10" and system 52" accomplish thismode of operation.

The torch 10" has generally the same construction as the torches 10 and10', except that the torch is serviced by two separate gas feed lines,each with its own solenoid valve, which feed separate plenum chambersand in turn feed separate, independent inlet holes in the swirl ring 28"or an equivalent structure. In the preferred form shown, there is afirst gas flow 44c which passes through the inlet tube 46", the valve48", an annular plenum chamber 42", the inlet holes 30", the plasmachamber 32" and out the exit nozzle orifice 16". This gas flow pathprovides a swirling gas flow for cutting that produces a good qualitycut and stabilizes the arc. The swirl is established by the holes 30"which are radially offset as is best seen in FIG. 3B. The plenum chamber42" is defined by the same components as in previous embodiment exceptthat the current ring has an annular downwardly extending wall 38a and aflange 38b at the lower edge that abuts a step recess in the swirl ring28" in a gas tight seal. (It will be understood that the seal can besecured with O-rings, a labyrinth seal, or any conventional gas sealthat also allows the swirl ring to be disassembled from the torch asnecessary.) The wall 38a and flange 38b separate and isolate from oneanother the outer plenum chamber 42" and an inner plenum chamber 42d".

A gas flow 44d passes through an inlet tube 46d", a valve 48d", theplenum chamber 42d", inlet holes 30d", the plasma arc chamber 42" andout the exit nozzle orifice 16". This second gas flow path for the flow44d uses inlet holes 30d" in the swirl ring that are generally radiallydirected as is best seen in FIG. 3C. The gas flow through the plasmachamber is therefore generally axial (downward) through the chamber 32"to the exit 16"; there is substantially no swirl.

In accordance with this process embodiment of the present invention, ata predetermined but very brief interval Δt before the current to thetorch is cut off, the controller closes the valve 48" for the flow 14cand opens the valve 48d" for the flow 44d. The interval Δt for a MAX 100brand torch operating with a reacting gas is typically less than 500milliseconds. This change in flow pattern, with no other changes in flowor current parameters, has also been found to provide dramaticreductions in electrode wear. However, this embodiment can be combinedwith the mass flow rate reduction embodiment described above withrespect to FIGS. 1A-1D and 2A-2C. For example, the current level can beramped down after t₁.

FIG. 3D shows a suitable system 52" for practicing the invention in thisaltered flow pattern mode. A control console 64" remote from the torchand therefore the substantial electromagnetic interference produced bythe torch, controls the gas flow 44c. A like console 64a" controls theflow 44d". In practice the consoles 64" and 64a" can be a single unit.

While various time periods have been suggested above for Δt, the optimalinterval will depend on the specific torch, its applications, andrelated parameters. In general, Δt is a function of the type of gas, thecurrent level, size of nozzle orifice, inlet flow area of swirl ring,the gas pressure, the gas flow rate, the gas flow pattern, and thephysical separation between the solenoid valves in the inlet and venttubes and the plasma. The separation is preferably less than 12 inchesfor the MAX 100 brand torch. This separation avoids delays andunintended variations in flow parameters due to the presence of a largefluid mass upstream of a plasma chamber and downstream of the valve. Thevalues for an acceptable interval Δt can readily be determinedempirically. Also, while the invention has focused on the alteration ofthe gas flow just prior to cut off, it should be understood that thealtered condition continues through electrical cut-off and for a briefperiod thereafter. However, the flow usually ceases entirely veryshortly after cut-off, whether due to a closing of the solenoid valve inthe inlet tube which eventually brings the flow through the plasmachamber to zero, or through a closing of the valve 48d" in the "radial"gas flow path to the radial hole 30d" in the FIG. 3A-3D embodiment.

There has been described a process and apparatus for reducing the wearon an electrode of a plasma arc torch, particularly a cutting torch, butin general for all types of plasma torches, e.g. ones for welding,spraying or other applications. The invention, in any of itsembodiments, can reduce the wear that presently occurs on all electrodesto an extent that the life of the electrode is at least doubled and canbe as much as ten times or more. The invention allows a given torch tobe operated at increased power levels and with reactive gases. Theseadvantages can be achieved with no dimunition of cut quality, and usingstandard electrodes and nozzles. In fact, due to the discovery that theswirling flow does not already affect the electrode wear during cuttingand knowing that a strong swirl yields high quality cuts, the swirlstrength can now be increased in torches to improve cut quality.Moreover, existing plasma arc torches and complete torch systems can bereadily modified to use the present invention.

While the invention has been described with respect to its preferredembodiments, it will be understood that various modifications andalterations will occur to those skilled in the art from the foregoingdetailed description and the accompanying drawings. For example, thetorch can have cooling passages not shown, be water cooled, or beadapted for welding or spray application of metal films. The torch canalso use a variety of starting techniques with corresponding changes inthe construction of the torch. The electrode and nozzle can assume avariety of configurations and constructions. The swirl ring 28" can beformed a two separate, axially stacked or axially spaced rings ratherthan as a single piece component as shown. These and other modificationsand alterations are intended to fall within the scope of the appendedclaims.

What is claimed is:
 1. A process for reducing the wear of an electrodeof a plasma arc torch having a torch body that mounts a nozzle at oneend of the torch adjacent a workpiece in a spaced relationship withrespect to the electrode to form a plasma chamber therebetween, meansfor directing a flow of an ionizable gas to the plasma chamber, andmeans for conducting an electrical current through the electrode toionize the gas to form the plasma arc, said flow directing meansproducing a swirling flow in said plasma chamber for at least a portionof the operation of the plasma arc, comprising,cutting off saidelectrical current at a time t₂ and altering the flow of the ionizablegas through said chamber at a time t₁ immediately before t₂, saidaltering maintaining a gas flow through said chamber prior to saidcutting off sufficient to control said arc, said altering alsocontinuing after said current cutting off, and said altering producing atermination of a swirling flow through said chamber generally coincidentwith said cutting off.
 2. The electrode wear reduction process of claim1 wherein said flow altering comprises a step selected from the group ofsteps consisting of (i) reducing the mass flow rate of the gas, (ii)reducing the swirl of the gas in the chamber, (iii) reducing the gaspressure in said chamber by reducing said electrical current just priorto said cutting off, and (iv) any combination of steps (i), (ii) and(iii).
 3. The process of claim 2 further comprising the step of ventingsaid plasma chamber in conjunction with said flow altering to alter theflow of the gas in the plasma chamber rapidly.
 4. The process of claims1, 2 or 3 wherein said altering occurs a sufficiently long time beforesaid cutting off to reduce electrode wear but a sufficiently short timebefore cutting off that the arc does not damage the nozzle.
 5. Theprocess of claim 4 wherein said altering reduces the pressure of saidgas in the plasma chamber generally exponentially and wherein saidaltering is initiated within about 500 milliseconds before said cuttingoff.
 6. The process of claim 2 wherein said reducing of the mass flowrate comprises closing the flow of gas through the torch body to theplasma chamber at time t₁.
 7. The process of claim 6 further comprisingspacing the point of said closing off close to said plasma chamber. 8.The process of claim 2 wherein said reducing of the current is gradualover the period commencing with said flow altering at t₁ and ending withsaid cutting off at t₁.
 9. The process of claim 8 wherein said currentreduction is ramped down to follow said reduction in the flow rate ofthe gas to the plasma chamber.
 10. The process of claim 8 wherein saidcurrent reduction is ramped down along with said reduction in the swirlof the gas flow in the plasma chamber.
 11. The process of claim 2wherein said reducing of the current commences prior to said flowaltering at t₁.
 12. The process of claim 2 wherein said reduction of theswirling includes switching from a flow into the plasma chamber that isradially offset to a flow into the chamber that is generally radial toproduce a substantially axial flow of the gas through the plasmachamber.
 13. The process of claim 12 wherein said altering is acombination of said switching to a generally radial flow and a generallyconcurrent reduction of said mass flow rate.
 14. The process of claim 12or 13 further comprising the step of venting the plasma chamber inconjunction with said switching to a generally radical flow.
 15. In aplasma arc torch system including a plasma arc torch having an electrodemounted in a body of the torch in a spaced relationship with respect toa nozzle having an exit port and mounted on the torch body at one end ofthe torch adjacent a workpiece to define a plasma chamber therebetweenthat is in fluid communication with the exit port and a first swirl ringalso mounted in the body that directs a flow of ionizable gas to theplasma chamber, and a power supply and means for conducting anelectrical current from the power supply to the electrode and via theplasma arc when it is in a transferred arc mode to the workpiece, saidpower supply including means for starting up and cutting off the currentto the torch, where the improvement comprisesmeans for changing oneparameter selected from the group consisting of (i) reducing the massflow rate of the gas through the plasma chamber, (ii) changing the flowpattern of the gas through the plasma to a radial flow and (iii) thecombination of the mass flow rate reduction and the gas flow patternchange, and means for operating said changing means immediately beforecutting off the current, whereby the wear of the electrode issubstantially reduced.
 16. The improvement of claim 15 wherein saidparameter changing means comprises a valve means for controlling theflow of the gas to the plasma chamber.
 17. The improvement of claim 16wherein said valve means closes to shut off the plasma gas flow to saidplasma arc chamber at time t₁ within about 300 milliseconds before saidcurrent cut off at time t₂.
 18. The improvement of claim 15 wherein saidparameter changing means comprises a second swirl ring with generallyradial flow ports for introducing gas to the plasma chamber mounted insaid torch body in combination with said first swirl ring and valvemeans for swithing the gas flow to said plasma chamber from said firstswirl ring to said second swirl ring.
 19. The improvement according toclaim 18 wherein said switching occurs within about 500 milliseconds ofsaid current cut off.
 20. The improvement of claim 15 wherein saidparameter changing means includes vent means from said plasma chamberand valve means operable under the control of said operating means foropening and closing said vent means.
 21. The improvement of claim 15wherein said change occurs within about 5 milliseconds of said currentcut off.